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Kirk D. Larson and Douglas V. Shaw

Three preplant soil fumigation treatments were applied on 5 Apr. 1993 to a nursery site that had not been planted previously to strawberries (Fragaria ×ananassa Duch.): 1) a mixture of 67 methyl bromide: 33 chloropicrin (CP) (by weight, 392 kg·ha–1) (MBCP); 2) 140 kg CP/ha; and 3) nonfumigation (NF). On 26 Apr., cold-stored `Chandler' and `Selva' strawberry plants of registered stock were established in each treatment. Soil and root/crown disease symptoms were absent in all treatments during the course of the study. In October, runner plants were machine-harvested and graded to commercial standards. The cultivars produced a similar number of runners per mother plant. Fumigation with MBCP, CP, and NF resulted in 18.56, 15.75, and 7.89 runners per mother plant, respectively. For `Selva', runner root and crown dry weights were similar for the MBCP and CP treatments, but NF resulted in significant reductions compared to the other two treatments. For `Chandler', fumigation with CP resulted in reduced root dry weight, and NF resulted in reduced crown and root dry weights compared to fumigation with MBCP. The results demonstrate the marked decreases in strawberry runner production and runner size that can occur in the absence of preplant soil fumigation, even on new strawberry ground. Also, small, but significant, reductions in runner production and runner size may occur with CP applied at a rate of 140 kg·ha–1 compared to standard fumigation with MBCP. Chemical name used: trichloronitromethane (chloropicrin).

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Kirk D. Larson and Douglas V. Shaw

Performance characteristics for 12 strawberry genotypes (Fragaria ×ananassa Duch.) from the Univ. of California, Davis, strawberry improvement program were evaluated in annual hill culture, with and without preplant soil fumigation using a mixture of 67 methyl bromide:33 chloropicrin (trichloronitromethane) (wt/wt, 392 kg·ha-1). Plants were established at two locations; one trial followed several cycles of strawberry plantation, whereas the other had not been cropped with strawberries for 20 years. Plant mortality was <3% and did not differ between soil treatments; thus, the main effects of fumigation treatment in these experiments were due to sublethal effects of soil organisms. Plants grown in nonfumigated soil produced 51% and 57% of the fruit yield of plants grown in fumigated soil for soils with and without a recent history of strawberry cultivation, respectively. Nonfumigated treatments also had reduced fruit weight and uniformly lower vegetative vigor during the early phases of plantation establishment. Significant genotype x fumigation interactions were not detected for any of the growth or performance traits at either location. Further, the proportion of variance attributable to interactions was at most 25% of that due to variation among genotypes, even for this highly selected population. Genotypic correlations for traits evaluated in different fumigation treatments ranged from 0.80 to 1.00; thus, selection in either soil environment is expected to affect largely the same sets of genes. These results demonstrate that strawberry productivity is substantially increased by fumigation, even in the absence of lethal pathogens or a discernible replant problem. More importantly, there appears to be little opportunity for developing cultivars specifically adapted to sublethal effects of nonfumigated soils.

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Kirk D. Larson and Douglas V. Shaw

Bare-rooted `Camarosa' strawberry runner plants were established in a fruit production field on 1 Nov. 1993 using annual hill culture and two preplant soil fumigation treatments: 1) a mixture of 2 methyl bromide: 1 chloropicrin (wt: wt, 392 kg·ha-1) injected into the soil before forming raised planting beds (MBC); or 2) nonfumigation (NF). At about 33-day intervals between mid-January and the end of May, 20 plants were destructively sampled from each treatment to determine leaf dry mass (LDM), crown dry mass (CDM), root dry mass (RDM), and shoot: root dry mass (SRDM) ratios. Plant mortality was <0.2% throughout the study and did not differ with soil treatment. Regardless of sampling date, LDM, CDM, and RDM were greater for MBC plants than for NF plants, although treatment differences were not always significant. During the first 143 days, NF plants allocated a greater proportion of dry matter to roots than to shoots compared to MBC plants, indicating that roots are a stronger sink for photoassimilate in nonfumigated than in fumigated soils. However, there was no difference between treatments in SRDM by the end of the study. Fruit yield and a 10-fruit weight were determined at weekly intervals from mid-January until 23 May 1994. Yield and mean fruit weight of NF plants were 72% and 90%, respectively, of that of MBC plants. For both treatments, about one-half of total fruit production occurred between 144 and 174 days after planting (late March to late April). During that same period, rates of dry matter accumulation in leaf, crown, and root tissues decreased for plants in both treatments, but greatest reductions occurred in NF plants. Chemical name used: trichloronitromethane (chloropicrin).

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Kirk D. Larson and Douglas V. Shaw

Strawberry (Fragaria ×ananassa L.) runner plant production during a 4-year period was compared on nursery soils treated with methyl bromide (MB) and chloropicrin (CP) mixtures (MB:CP) and three alternative soil treatments: CP, mixtures of 1,3-dichloropropene (Telone®) and CP (DP:CP), and no fumigation (NF). The effect of soil treatment on runner plant production for a single nursery propagation cycle was determined in all 4 years. In 2 years, runner production in a final propagation cycle was also determined as a function of soil treatment in previous cycles. A single propagation cycle in NF soil decreased runner production relative to all other treatments. Treatments with CP at rates of 140 to 191 kg·ha–1 generally decreased runner production significantly (P ≤ 0.05) in comparison with treatment with MB:CP; use of CP at rates ≥303 kg·ha–1 resulted in statistically equivalent runner production. In one trial, use of two DP:CP formulations (516 kg·ha–1 of a 7:3 DP:CP mixture, and 448 kg·ha–1 of a 3:7 DP:CP mixture) significantly reduced and did not affect runner production, respectively, relative to the use of MB:CP. Use of MB:CP in the previous propagation cycle also increased runner productivity in comparison with NF. Runner productivity of planting stock produced with 314 kg·ha–1 of CP did not differ statistically from that of stock produced with MB:CP, but productivity of planting stock on soil treated with 157 kg·ha–1 of CP was intermediate between that on NF and MB:CP-treated soil. Planting stock grown on nontreated soil in two previous propagation cycles produced 25% fewer runner plants than did similar stock grown on MB:CP-treated soil. Productivity of planting stock produced with CP at rates of 280 to 314 kg·ha–1 in two previous propagation cycles did not differ statistically from that of stock produced with MB:CP. Results of meta-analyses indicated that fumigation with MB:CP was more effective in increasing runner production than was CP or NF, regardless of the propagation cycle or rate of application. For mixtures of 1,3-dichloropropene and CP, nursery productivity was maximized by using at least 280 kg·ha–1 of CP.

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B. de los Santos, C. Barrau, C. Blanco, F. Arroyo, M. Porras, J.J. Medina and F. Romero

Several preplant soil fumigation treatments were repeated over a period of three years on strawberry (Fragaria ×ananassa Duchesne) crops, at two different places in the province of Huelva (southwestern Spain). The influence of these treatments on Trichoderma soil populations and on populations of soilborne pathogens was examined every year by isolating soil onto selective media. No strawberry pathogens were detected but Trichoderma soil populations increased each year after the treatment. Significant differences were noted between the treatments and also compared to the control. The largest populations were observed after treatments with methyl bromide and chloropicrin, and so resulting in a higher production. Chemical name used: trichloronitromethane (chloropicrin).

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S.D. Nelson, S.J. Locascio, L.H. Allen Jr., D.W. Dickson and D.J. Mitchell

Methyl bromide (MeBr) is an important and effective soil fumigant commonly used to control weeds and soilborne pests. Because MeBr has been implicated as a contributor to the depletion of stratospheric ozone, it is scheduled for phaseout by 2005. This study examined nonchemical and chemical practices as alternatives to MeBr. Off-season flooding followed by a series of soil preplant chemical treatments [MeBr with 33% Pic; 1,3-D mixed with 17% (C-17) and 35% (C-35) Pic combined with Peb; and metam-Na combined with 1,3-D and Peb were evaluated on spring tomato (Lycopersicon esculentum Mill.) and eggplant (Solanum melongena) production in northern Florida. Pest control and tomato and eggplant yields were not significantly different between the flooded and non-flooded control plots. The most effective alternatives to MeBr were 1,3-D and Pic mixtures (C-17 and C-35) combined with Peb. Tomato and eggplant yields for these chemicals were statistically equivalent to that of MeBr. Tomato, but not eggplant, yield and nematode control were poor with metam-Na combined with 1,3-D and Peb in comparison to the other fumigant combinations. Chemical names used: 1,3-dichloropropene (1,3-D); trichloronitromethane [chloropicrin (Pic)]; S-propyl butyl(ethyl)thiocarbamate [pebulate (Peb)]; sodium N-methyldithiocarbamate (metam-sodium (metam-Na)].

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Douglas V. Shaw and Kirk D. Larson

Yield and fruit size were determined for 49 strawberry (Fragari ×ananassa Duch.) genotypes during a 7 year period, in soils prepared with and without preplant soil fumigation using 2 methyl bromide: 1 chloropicrin (wt/wt). Strawberries were grown in alternate years, with the nonfumigated treatment representing the first, second, third, and fourth strawberry crop cycles initiated without soil fumigation. Highly significant (P < 0.01) effects of soil fumigation treatment were present for yield in a combined analysis over all years; fumigation increased yield by 41% over nonfumigated soils in the first nonfumigated cultivation cycle and by 68% to 74% for subsequent nonfumigated cycles. Fruit size was less affected by soil treatment but increases due to fumigation (2% to 18%) were significant (P < 0.05) in the third or fourth nonfumigated crop cycle. Genotypic variances were highly significant in the combined analysis, whereas geneti × fumigation interaction variances were significant only for fruit size and contributed <8% of the total phenotypic variance for either trait. Genetic correlations were r g = 0.77 and 0.92, respectively, for yield and fruit size treated as independent traits across soil fumigation environments. There was no evidence for genes that confer specific adaptation to nonfumigated soils, or that these genes emerge as important contributors to the phenotypic variation as the soil environment deteriorates with repeated cultivation of strawberry in nonfumigated soil. Chemical names used: trichloronitromethane (chloropicrin).

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Kirk D. Larson, Douglas V. Shaw and Jerry Sterrett

Three preplant soil fumigation treatments were applied to a strawberry fruit production field in Summer 1993: 1) a mixture of 67 methyl bromide: 33 chloropicrin (wt/wt, 392 kg·ha–1) (MBC); 2) chloropicrin (trichloronitromethane, 336 kg·ha–1) followed by metam sodium (935 liters·ha–1) CMS); and 3) nonfumigation (NF). Bare-rooted `Camarosa' strawberry plants were established in each treatment on 1 Nov. in annual hill culture. Plant mortality was <1%; thus, differences in growth and productivity among treatments were due to sublethal effects of competitive soil organisms. Fruit yields were recorded weekly from 14 Jan. to 23 May 1994. For the NF treatment, early season (January–March), late season (April–May), and total yields were 86%, 69%, and 72%, respectively, of those of the MBC treatment. Early season yields were greatest for the MBC treatment, but late and total yields were greatest for the CMS treatment. From Jan. through May 1994, 20 plants were destructively harvested from each treatment at about monthly intervals for determination of leaf (LDW), crown (CDW), and root dry weight (RDW). For a given date, LDW, CDW, and RDW of plants in the MBC and CMS treatments were greater than those of the NF plants. From January to March, plants in the NF treatment allocated a proportionally greater amount of dry matter to roots, and proportionally less dry matter to crowns and leaves than fumigated plants. In April and May, root: shoot ratios were similar for all three treatments. These data demonstrate the marked influence of soil fumigation treatment on yield and dry matter partitioning of strawberry, and suggest that combinations of chloropicrin and metam sodium may be a viable, albeit expensive, alternative to fumigation with methyl bromide.

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Kirk D. Larson

Replant soil fumigation with mixtures of methyl bromide (MeBr) and chloropicrin (trichloronitromethane) is a standard practice for pest and disease control in fruit crop nurseries in California. The proposed phase-out of MeBr by the year 2001 requires that alternative soil sterilants be studied for nursery use. Therefore, on 5 April, 1993, three preplant soil treatments were applied to new strawberry ground: 1) MeBr/chloropicrin (67:33) at 392 kg/ha: 2) chloropicrin, a possible MeBr substitute. at 140 kg/ha: and 3) nonfumigation. The experimental design was a RCB: there were two plots (each 10′ × 15′) for each of two cultivars (`Chandler' and `Selva') for the 3 soil treatments in each of 3 blocks. Mother plants were planted 26 April, and plots were machine-harvested in October, 1993. All plants from each plot were uniformly graded, after which mean stolon yield per mother plant, mean crown diameters, and crown and root dry wts were determined. Cultivar effects and cultivar × treatment interactions were not observed, so data for the two cultivars were pooled. Stolon production per mother plant was greatest for trt 1 (18.56 stolons), intermediate for trt 2 (15.75 stolons), and least form 3 (7.89 stolons). For trt 3, crown dieters. and crown and root dry wts were reduced relative to those of trts 1 or 2. Stolons from all trts were planted in a fruit production field on 13 October, 1993. After two months, canopy diameters were greatest for plants from trt 1 (27.1 cm), intermediate for plants from trt 2 (26.2 cm) and least for plants from trt 3 (24.9 cm). The results indicate that, compared to standard soil fumigation with MeBr/chloropicrin. small, but significant, reductions in runner production and plant vigor can be expected following nursery soil fumigation with intermediate rates of chloropicrin.

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Douglas V. Shaw, Thomas R. Gordon and Kirk D. Larson

Strawberry runner plants from the cultivar `Selva' (Fragaria ×ananassa Duch.) were produced using three nursery treatments in each of three years: propagation in soil fumigated with a mixture of 2 methyl bromide: 1 chloropicrin (w/w) at 392 kg·ha-1, propagation in fumigated soil but using planting stock inoculated prior to nursery establishment with a conidial suspension of Verticillium dahliae (106 conidia/mL), and propagation in nonfumigated soil naturally infested with V. dahliae. Runner plants were harvested and stored at 1 °C for 6, 18, or 34 days prior to establishment in fruit production trials. No significant differences were found between runner plants grown in naturally infested soil and runner plants obtained from artificially inoculated mother plants for V. dahliae infection rates detected by petiole isolation immediately prior to transplanting, the percentage of plants visibly stunted due to disease during the following production season, and seasonal yield compared with corresponding noninfected controls. Cold storage of runner plants for 18 or 34 days, produced using either natural or artificial inoculation systems, reduced the initial percentage of infected plants by 42% to 61% and the percentage of stunted plants during the following fruit production season by 43% to 57%, compared with plants from corresponding nursery treatments given only 6 days post-nursery cold storage. Yields for inoculated plants with 6 days cold storage were 16% to 20% less than those for uninoculated controls, whereas yields for inoculated plants with 18 or 34 days of storage were 3% to 9% less than the respective controls. Most of the cold storage effects on initial infection rate, stunting, and yield were realized at the 18 days of storage treatment. A reduction in the fraction of V. dahliae infected plants due to cold storage, suggests either a direct effect of cold storage on the disease organism or stimulation of secondary resistance mechanisms in the plant. Chemical name used: trichloronitromethane (chloropicrin).